Undoubtedly the most thoroughly explored and widely used catalyst for the hydrodesulfurization of mineral oils and fractions thereof is of the cobalt-molybdenum-alumina type, hereinafter referred to as "CMA" catalysts. Several methods have been developd and utilized for the manufacture of CMA catalysts, the most common probably being impregnation of preformed gamma alumina supports with aqueous solutions of ammonium molybdate and a soluble cobalt salt, either simultaneously or sequentially, followed by calcining at temperatures ranging between about 900.degree. and 1400.degree. F. An obvious principal objective in the formulation and manufacture of such catalysts is to develop the highest possible desulfurization activity per unit volume of catalyst, consistent with economic limitations.
To develop maximum activity in CMA catalysts, it would be difficult to conceive of a more obvious avenue to explore than that of increasing the content of active metals in the catalyst. However, due to various limitations such as pore volume, pore size and surface area of the alumina support, as well as other more imponderable factors, it has not yet been found possible to derive worthwhile benefit from increasing the metal loading of CMA catalysts beyond certain rather definite levels, usually between about 18-20 weight-percent MoO.sub.3 and 4-5 weight-percent CoO. As a consequence virtually all commercial CMA catalysts contain between about 12 and 16 weight-percent MoO.sub.3 and about 2-5 weight-percent CoO. In commercially prepared CMA catalysts, activity usually begins to level off at about 16 weight-percent MoO.sub.3, and at higher MoO.sub.3 levels, above about 22 weight-percent, a very definite loss in activity is usually observed. The foregoing is aptly illustrated by the following Example.